1. Field of the Invention
This invention relates to a laminated film suitable for photosensitive materials, particularly for photographic photosensitive materials.
2. Description of the Prior Art
Various types of laminated films used for packaging materials for photosensitive materials have been put to practical use, and various properties are required according to their uses.
As a packaging material for photosensitive materials which lose the value by exposing them to light, laminated films capable of shielding light completely are used. The properties required are packaging material suitability, light-shielding, gas barrier, moistureproofness, antistatic property, rigidity, physical strength such as breaking strength, tear strength, impact puncture strength, Gelbo test strength and wear resistance, heat sealing properties such as heat seal strength, hot tack properties (hot-seal ability) and seal ability of contraries, flatness, low disting characteristics slipping character and the like.
Heretofore, the packaging materials utilized are a high pressure branched low density polyethylene (LDPE) resin film blended with carbon black, a laminated film composed of a LDPE resin film, a paper and an aluminum foil. The laminated film was, as shown in FIG. 8, composed of a LDPE resin film layer 12a containing a light-shielding material, an aluminum foil 11 and a bleached kraft paper 10 each through an adhesive layer 6.
Besides, the inventor has already developed a laminated film composed of a cross-laminated film using uniaxially stretched high density polyethylene (HDPE) resin films excellent in physical strength and a LDPE resin film containing at least one of a light-shielding material and an antistatic agent laminated thereon, as a packaging material, particularly a sealed lightproof bag, for a roll of a photographic photosensitive material, weight sheet-shaped photosensitive materials of more than 1 kg (U.S. Pat. No. 4,147,291, Japanese Utility Model KOKOKU No. 56-19087). Another packaging material developed by the inventor has an inexpensive cross-laminated film improved in heat sealing properties and light-shielding ability U.S. Pat. No. 4,258,848, Japanese Utility Model KOKOKU No. 61-20590 . Another packaging material developed by the inventor is a laminated film having a linear low density polyethylene (L-LDPE) resin layer containing carbon black being excellent in physical strength, such as tear strength and impact puncture strength, and heat sealing properties and inexpensive (U.S. Pat. No. 4,452,846, Japanese Patent KOKAI No. 58-132555). As a packaging material using a metallized film layer, the inventor has developed a packaging material composed of a metallized film layer and two L-LDPE resin polymer layers containing more than 50 wt.% of L-LDPE resin disposed on both sides of the metallized film layer (U.S. Pat. No. 4,663,218). Either or both of the L-LDPE resin polymer layers contains 0.3 to 30 wt. % of a light-shielding material.
On the other hand, the laminated films for packaging foods widely used are light packaging bags composed of an aluminum vacuum-metallized biaxially stretched film coated with a hot melt adhesive layer or the like as the heat sealing layer used mainly for snacks.
In the aforementioned conventional laminated film having a LDPE resin film, physical strength particularly tear strength is weak, even though the thickness is thick, and curling is great. Moreover, heat sealing properties are inferior, and it is expensive. As a result, during packaging works and transportation, various problems occur such as dusting, puncture, torn and the separation of heat sealed portion.
The packaging material having a cross-laminated film disclosed in U.S. Pat. No. 4,147,291, U.S. Pat. No. 4,258,848 are excellent as the packaging material for weight materials because physical strength such as tear strength and tensile is remarkably improved. Therefore, this packaging material was put to practical use up to recently. However, since an uniaxially stretched HDPE resin film is used as the heat sealing layer, heat sealing properties are inferior. Curling or variation in physical strength occurred caused by the unevenness of the thickness of the adhesive layer or of the draw ratio of the uniaxially stretched HDPE resin film. Therefore, the package was occasionally separated or punctured. In addition, since two kinds of film molding machines are necessary to the production of the cross-laminated film where a longitudinally uniaxially stretched film and a laterally uniaxially stretched film are laminated so as to cross each other, this laminated film is expensive.
Since the packaging materials having a light-shielding L-LDPE resin film layer disclosed in U.S. Pat. 4,452,846 or Japanese Patent KOKAI No. 58-132555 are inexpensive and excellent in heat sealing properties and physical strength such as tear strength and impact puncture strength, they are excellent as the packaging material for photosensitive materials. However, in the case of packaging a weight photosensitive material or a photosensitive material having sharp edges, the light-shielding L-LDPE resin films were occasionally elongated and made thin due to their low Young's modulus, though they were not punctured nor torn. In this case, light-shielding and moistureproofness cannot be secured sufficiently.
The packaging material disclosed in U.S. Pat. No. 4,663,218 is improved in physical strength such as tear strength. However, when only one of the L-LDPE resin polymer layers contained a light-shielding material, the curling of the packaging material was great. When the melting points of the L-LDPE resin polymer layers were almost the same as each other, the outside layer was melted at the time of heat sealing, unless the heat sealer was provided with a special processing. As a result, not only pinholes and the degradation of physical strength occur, but also appearance became worse. Therefore, they were put to practical use as a laminated film, laminated to a high Young's modulus heat-resistant film such a aluminum vacuum metallized nylon or polyester film. However, the curling of the laminated film was great, and moreover, the above nylon film and polyester film were expensive.
As to the laminated film for packaging snacks, the gloss was too much, and the physical strength was weak. The curling was great, and the heat seal strength was small.